Traditional magnetic coil ballasts possess a number of operational disadvantages, such as poor energy efficiency and high visible flicker. Electronic ballasts overcome many of the shortcomings of magnetic ballasts, but at a considerably higher monetary cost.
A common type of electronic ballast includes a rectifier circuit, a switching converter for providing power factor correction, a high frequency inverter, and an output circuit. Such a ballast provides a high frequency current for driving the lamps with minimal visible flicker and is far superior to magnetic ballasts with regard to energy efficiency and power factor correction. On the other hand, such a ballast typically requires three or more power transistor switches, in addition to a large number of other components, of which electrolytic capacitors and magnetic components such as inductors and transformers are typically the most costly and the most difficult to manufacture. Due to its complexity and high component count, the resulting ballast is not economically competitive with relatively low cost magnetic ballasts.
In addition to the drawback of cost, several types of electronic ballasts also possess the important disadvantage of significant in-rush current. In-rush current, which is an inherent characteristic of many electronic circuits which have a large bulk capacitance, is a transient pulse of current that is generated when power is first applied to the circuit. The amplitude of the in-rush current pulse is maximized when power is first applied to the circuit at the peak of the AC line voltage cycle. The peak value of the high current pulse drawn by the circuit from the AC line source in such a case is customarily referred to as the peak in-rush current.
Excessive in-rush current is highly undesirable, having been associated with nuisance tripping of circuit breakers as well as degradation and welding of switch contacts on AC line-side equipment such as relays and occupancy sensors. An additional disadvantage of high in-rush current is the resulting design requirement of high surge current ratings for those circuit components through which the in-rush pulse flows.
Further, many electronic ballasts include one or more energy storage capacitors, and contain a switching converter in which the voltage across the energy storage capacitor(s) appreciably exceeds the peak value of the AC line voltage. Due to several operational and performance requirements, the energy storage capacitors must have a relatively large capacitance value which, when combined with the need for a relatively high voltage rating, dictates the use of electrolytic capacitors. Since the monetary cost and physical size of an electrolytic capacitor increases with the arithmetic product of its capacitance and its voltage rating, a substantial reduction in the material cost and physical size of the ballast can be realized by developing a ballast having a converter stage with a significantly lower voltage across the energy storage capacitor(s).
Thus, a need exists for an electronic ballast circuit that rivals the low monetary cost and low in-rush current of magnetic ballasts, but that retains at least some of the key advantages, such as high energy efficiency and negligible visible flicker, of more costly electronic ballasts. Since magnetic components, power transistor switches, and electrolytic capacitors are among the largest and most expensive parts used in electronic ballasts, and thus detract greatly from the goals of low material and manufacturing cost, significant impetus exists for developing new ballasts in which the number, complexity, and cost of such components is reduced or minimized.
It is therefore apparent that an electronic ballast which provides energy efficient, low flicker, high frequency powering of fluorescent lamps, which has low in-rush current, and which requires fewer and less costly components than existing electronic ballasts, would constitute a considerable improvement over the prior art.